The present invention generally relates to dose control in continuous wave and excimer laser based lithography tools.
Typically, semiconductors wafers in integrated circuit fabrication are exposed to irradiation of a light source in an exposure tool. In the exposure tool the irradiation light passes through an optical column with optical elements such as spherical and aspheric collimator lenses and blinds. After the optical column the light traverses a reticle with device patterns on it and the device patterns are projected onto a wafer below. The wafer is covered by a photoresist. The chemical composition of the photoresist is changed by the irradiation with light and a latent image is formed in the photoresist. After irradiation of a first target area either the wafer is moved from a first position to a second position with the optical column being fixed or the optical column is moved from a first position to a second position with the wafer being fixed, and a second target area on the wafer is irradiated. Finally, the wafer is moved to another tool where the photoresist is developed and partly removed so as to form a protection layer including windows on the wafer. The protection layer corresponds to the device pattern on the reticle, and etchants react with the semiconductor wafer or ions are implanted into the semiconductor wafer through the windows of the protection layer.
The light source in the exposure tool may be a pulsed light source or a continuous wave light source. In case of a pulsed light source typically multiple pulses are necessary to form a complete projection of the device pattern in the photoresist layer on the semiconductor wafer. For instance, a single laser pulse 301 may have a free running output of approximately 250-300 millijoules (mJ) per pulse. However, only a very small portion of the output is used for exposure, namely a narrow band of frequency that may contain approximately 5% of the total energy of the light output of the laser.
Typically, the energy of the laser pulses varies. Since however the total amount of energy being absorbed by the photoresist must be the same in each irradiated target areas in order to ensure critical dimension (CD) uniformity, there are xe2x80x9caveragingxe2x80x9d means employed between the laser source and the wafer in order to obtain an exposure energy in the photoresist that is substantially equal for all target areas. As xe2x80x9caveragingxe2x80x9d means attenuator filters are used that reduce the power per pulse. For instance, 50 pulses are used to expose the photoresist in one target area. This is a greater number of pulses than is necessary for a complete exposure dose. However, running the exposure with more but less intensive laser pulses allows a dose control within 1% despite variations in energy output from pulse to pulse.
Pulse stability in the current version of deep ultraviolet (DUV) excimer lasers has improved significantly since their introduction. However the next generation lasers including argon fluoride (ArF) and fluoride (F2) lasers will have less pulse stability until this laser technology matures which is to be expected to be several device technology generations away. It would be time consuming and costly to change mask filters with each variation of the light source.
It is therefore desirable to provide a means that can easily be adapted to the varying requirements of averaging the pulse power of different light sources.
The present invention seeks to provide an optimized exposure tool which reduces the number of processing steps and complexity of the prior art.